Hello all. New member here. Having a braking issue I just cannot figure out:

At a cruise event I noticed my brakes were getting spongy in the heat with the stop and go. I pressed hard, felt a thunk in the pedal and the brake light came on.

Pulled over, added some fluid and had braking still and everything felt fine, just light on. Was able to drive home a few hundred miles and didn't notice anything really all that different from before the light went on while driving a couple hundred miles to the cruise.

Looked up the issue in 1969 Ford Shop Manual and followed the process to reset the distribution block piston. Felt the piston pop back and the light went off.

Turned the car on and the brake pedal became an instant sponge and almost literally fell to floor with barely a touch, and came to a hard stop about 4-6 inches of travel toward the floor.

Figured I must have a bad master cylinder so ordered a reman unit. Bench bled it, bled the lines. Pedal pumped up nice and hard when engine was off. Turned the car on and the exact same thing - the brake pedal felt like absolutely no resistance and fell down on very light pressure until it stopped about 4-6 inches down from the resting pedal postioin. Brake pedal would not pump up and the brakes will only barely stop the car at an idling roll. No way they could stop the car under normal driving.

Seemed like I had a bit of back brakes, but almost no front brakes. Thought maybe the prop valve was bad so put on an aftermarket adjustable unit. Still the same.

Re-bled the master cylinder and the lines. Still the same.

Thought maybe the distribution block had a blockage or something from the piston moving, so disassembled, cleaned and rebuilt with the kit and directions from West Coast Cougars.

Bench bled the master cylinder again just in case. Reassembled. Brakes pumped up fine after 4 or 5 pedal strokes. Pedal was hard. Started the car up and bam, the pedal went soft with almost no resistance just like the last few times.

Only thing I noticed was that the master cylinder seemed to actuate very late during the stroke while bench bleeding.

At a complete loss of what the root cause is for this could be. Why brakes will pump up when the engine is off, but soft when the engine is on. Doesn't seem logical that 2 master cylinders would be bad, but maybe should try another unit?

Not able to check if the brakes are actually working when the engine is off, but again, they BARELY work at all when the engine is on.

Any help would be greatly appreciated.

 

its using a vac assist system? as in they are power brakes?

you talked about bleeding the MC a bunch but what about the slave cylinders...Did you bleed them? Did you see air come out?
When bleeding the SCs did you notice any apparent brake fluid leaks around them?

The push rod that goes from the pedal into the MC have much extra travel? If you pulled the brake pedal up towards you does it move much? with it pulled all the way up how far down does it move before you feel the push rod contact the MC piston?

What brand is the new MC?...its totally possible that is a POS too so dont rule out that its good just because its new.

 

Hi Gun,

I assume by slave cylinder you mean the front calipers and rear cylinders? If so, yes.

They are power brakes.

The push rod seems fine. When the car is off they pump up fine and there is no lift from there. But then I turn the car on and the pedal no longer has any feel. It is just like pushing a clutch pedal until it reaches about 1/2 inch before the full travel of the pedal and then you get a slight engagement of braking. But not enough to actually brake the car.

The pedal will return when you release.

Any idea what the bleeder is for the is on the front of the MC? I haven't seen one with a bleeder there before.

 

There shouldn't be a port on the front of the MC. If there is it is either 1) for a different vehicle 2)a generic MC that also has applications with the brake light operating off that port (not the pedal). Sounds like something is wrong with the MC to me, or perhaps the pushrod is adjusted incorrectly in between the pedal to booster or booster to MC

 

The original master cylinder had a bleeder on the front as well?

I have adjusted the pushrod all the way out and in the process of completing the bleeding process after removing the master cylinder to do that.

 

I don't believe there was a bleeder port on them originally, at least I haven't seen it that way. I could be wrong though.

 

If it all feels good until the vacuum assist comes into play when the car turns on then I think we have our culprit. Take the vacuum assist out of the equation and see how it feels.

 

For your own peace of mind, it is easy to elliminate the effects of the engine running on the brake system, simply unhook the vacuum line going to the brake booster. And plug the line so you don't creat a huge vacuum leak. Done.
have you physically checked the rear brake drums? Pull the drums off? When it all first started, it was suddenly and the light came on. That tells me one side of the system ( front or back ) lost pressure. If that happens it is a leak or maybe something failed in the rear brakes allowing too much movement of the shoes. Never heard of that happening but but worth checking. If pressure was lost, the leak was either internal( master CYL, no fluid lost) or external, usually at the wheel cylinders, or maybe a break line and fluid is lost. Brake hoses have done some weird things too, splitting internally , no leaks, just partial blockage and could perciecably intermitant too. Do you know how old the lines are? Remember that there is one between the body and the rear end too.
i always try to buy new parts and not rebuilt, so much of them are junk now!!
dont get me started!! Anyway stuff to check out, keep us updated👍
Don